The present invention relates to optical fibers, and in particular to optical fibers which are installed in contact with or closely adjacent to high voltage electrical equipment, such as high voltage power conductors.
The number of applications of optical fibers on overhead high voltage conductors is growing at an increasing rate as the general use of fiber optic systems continues to increase at a rapid rate. High voltage applications are generally associated with fiber optic devices such as current, voltage and temperature sensors installed on an electrical network to monitor the network operating conditions. In addition, fiber optic cables are often attached to high voltage conductors and used for long distance voice, video and data communications.
Although optical fibers are inherently good dielectric materials, they still constitute a threat to the integrity of the high voltage power system on which they are installed if proper precautions are not taken at all transition points where the fiber optic system is connected to electrically grounded opto-electronic signal processing equipment. A particular problem encountered when taking a fiber optic system from a high voltage potential to a ground potential is dielectric tracking. Dielectric tracking occurs when electric leakage currents flowing on the surface of an insulating material cause surface temperatures to rise to a level at which degradation of the material occurs. Dielectric tracking problems are increased by environmental conditions such as rain, fog, salt spray, dust and numerous industrial pollutants. Once dielectric tracking has been initiated, it tends to perpetuate until the dielectric strength of the insulating system is sufficiently reduced to cause dielectric failure of the system, usually by line to ground flashover.
The coatings, buffers and jackets used on optical fibers are primarily formulated and designed to enhance the handling and physical performance characteristics of the optical fibers with little or no consideration given to the dielectric tracking resistance for high voltage applications. It is therefore necessary that insulators specifically designed to support and protect the optical fibers must be used for high voltage applications.
Although the primary function of an insulator for high voltage applications is to provide a surface resistant to dielectric tracking with sufficient extended creepage length to prevent line to ground flashovers under inclement and contaminated conditions, there are other important performance criteria which must be met. In particular, in high voltage transmission line applications, distances of 10 to 25 feet between the ground and the high voltage line are common. It is thus important that the optical fibers are properly supported and contained to prevent excessive motion due to environmentally induced vibrations of the high voltage conductors and of the optical fibers.
The use of optical fibers in association with high voltage power conductors is known in the prior art. For example, U.S. Pat. No. 4,772,090 discloses an arrangement whereby a fiber optic cable may be routed through or around equipment at different electrical potentials, including ground potential. U.S. Pat. No. 4,717,237 discloses an overhead electric and optical transmission system in which the overhead electric conductor is mechanically secured to a support structure by a tension insulator having a through-bore for an optical fiber. U.S. Pat. No. 5,124,634 discloses an optical current transducer which uses an insulator pedestal as an optical fiber raceway.
The insulator systems of the prior art have disadvantages which would be desirable to overcome. In particular, ingress of moisture is a severe problem which will ultimately cause failure of an insulating system. Existing systems that have been modified for use with optical fibers rely on mechanical sealing mechanisms at each end of the insulator to prevent the ingress of moisture. Such mechanical seals are susceptible to failure after repeated thermal cycling. It would therefore be desirable to devise an insulation system which does not rely on mechanical seals. Also, the prior art insulators are typically made of ceramic or other heavy materials, making such insulators difficult to use in some situations. Further, prior art insulators are not easily adapted to different application configurations, such as varying line to ground distances. It would therefore be desirable to provide an insulation system which is easily adapted for differing applications, and which is lightweight as well.